The present invention relates to a method of diffusing aluminum into a silicon substrate for manufacturing a semiconductor device.
Usually, boron, aluminum or gallium is used as a P-type dopant in a silicon substrate. Aluminum among these dopants is most useful for the formation of a diffusion layer since aluminum has the highest rate of diffusion into silicon. Namely, the diffusion coefficient of aluminum in silicon is ten to twenty times as large as that of boron and several times as large as that of gallium. Therefore, the time required for forming a diffusion layer of a predetermined depth by use of aluminum is one-tenth to one-twentieth of that required in the case of using boron and a half to one-tenth of that required in the case of using gallium.
The aluminum dopant is also advantageous because of a low rate of occurrence of crystal lattice distortion in silicon and a small interaction with other dopants. However, the diffusion of aluminum into a silicon substrate has many questions upon which there are difficult in view of technical point. Especially, a highly accurate control of aluminum diffusion concentration is difficult. A typical conventional method of diffusing aluminum into a silicon substrate includes introducing the silicon substrate and an aluminum diffusion source into an evacuated heat-resistive chamber such as quartz and heating the chamber to diffuse evaporated aluminum of a gas phase into the silicon substrate.
The maximum diffusion concentration of aluminum in the silicon substrate is greatly changeable depending upon a vessel for holding the aluminum diffusion source. When a wire of aluminum having a purity of 99.995% is used as the diffusion source and alumina is used the diffusion source holding vessel material, the maximum aluminum diffusion concentration of 10.sup.19 atoms/cm.sup.3 can be obtained. However, when silicon or quartz is used as the holding vessel material, the maximum aluminum diffusion concentration is below 10.sup.17 atoms/cm.sup.3.
The maximum aluminum diffusion concentration in the silicon substrate is also greatly changeable depending upon the composition of the aluminum diffusion source. For example, in the case where a silicon-aluminum alloy is used as the diffusion source, the aluminum diffusion concentration of about 10.sup.19 atoms/cm.sup.3 can be obtained when the concentration of aluminum in the diffusion source is above 65% (i.e. the concentration of silicon is below 35%). But, when the aluminum concentration of aluminum in the diffusion source is below 60% (i.e. the concentration of silicon is above 40%), the obtained aluminum diffusion concentration would be in order of about 10.sup.16 atoms/cm.sup.3. Therefore, it is difficult to accurately control the diffusion concentration over a range of 10.sup.17 -10.sup.19 atoms/cm.sup.3.
In the vapour phase diffusion of aluminum within a quartz chamber, a part of the aluminum vapour is consumed by the reaction thereof with quartz of the chamber. When the quantity of the aluminum vapour in the quartz is sufficient, aluminum can be diffused into the silicon substrate with a concentration of about 10.sup.19 atoms/cm.sup.3 which is a limit at which aluminum is soluble in its solid state into silicon (i.e. the maximum solid-solubility of aluminum into silicon). However, the quantity of the aluminum vapour in the quartz chamber is not sufficient, aluminum can be merely diffused into the silicon substrate with a concentration of at highest about 10.sup.16 atoms/cm.sup.3. Thus, in the conventional vapour diffusion method, it is generally difficult to accurately obtain an aluminum diffusion concentration of 10.sup.17 -10.sup.19 atoms/cm.sup.3 which is practically useful.
A two-step diffusion technique using a preposition diffusion step and a drive-in diffusion step is known as a diffusion method which can provide a low diffusion concentration. In accordance with this technique, however, if the diffusion concentration at the preposition diffusion step is selected to be low, the fluctuation of diffusion concentration or diffusion depth becomes large, thereby making an accurate control of diffusion concentration over a wide range difficult. Therefore, there has been proposed a method in which a pattern of aluminum formed or deposited on a surface of a silicon substrate by means of evaporation techniques, etc. is used as a diffusion source. Such a method is described in U.S. Pat. No. 4,040,878 issued on Aug. 9, 1977. In accordance with this method, however, the aluminum surface concentration of the silicon substrate reaches the maximum solid-solubility of aluminum into silicon. As a result, it is difficult to form a low concentration diffusion layer in the silicon substrate. Further, it is difficult to accurately control the concentration of diffusion of aluminum into the silicon substrate in accordance with the thickness of the deposited aluminum pattern or layer, since the resultant diffusion concentration fluctuates. Furthermore, the deposited aluminum layer is oxidized during the thermal diffusion process to produce a surface layer of .alpha.-alumina so that the surface flatness of the silicon substrate is remarkably deteriorated. The .alpha.-alumina layer which is difficult of chemical dissolution, is undesirable.